The present invention relates generally to output stages for DC driver circuits having non-isolated outputs. More particularly, the invention relates to improving efficiency and reducing currents within an output stage of a DC driver circuit having a non-isolated output.
Generally, there are two types of output stages for direct current (DC) driver circuits (e.g., LED power supplies): isolated output and non-isolated output.
An isolated driver circuit (i.e., isolated output power supply) has an output transformer to isolate the output of the driver circuit from the input of the driver circuit. Because the controller is operating on the input side of the driver circuit and the output is isolated from the input, sensing voltage and current conditions in the output stage of the driver circuit at the controller and controlling the output of the output stage is relatively complicated (e.g., requires the use of operational amplifiers for isolation). Additionally, the efficiency of the driver circuit is reduced because power has to pass through the isolation transformer, and the isolation transformer itself adds significant size and cost to the driver circuit.
A non-isolated driver circuit (i.e., a non-isolated output power supply) simplifies output condition sensing and control, increases efficiency, and reduces the costs and size penalties imposed by the isolation transformer in isolated power supplies. However, output noise is an issue in non-isolated power supplies because a full bridge rectifier is typically used.
Referring to FIG. 1, a power source 102 provides power to a non-isolated DC power supply or driver circuit 100 with a full bridge rectifier. The non-isolated DC power supply 100 receives power from the power source 102 at an input stage 104. In one embodiment, the input stage 104 includes an alternating current (AC) to DC converter, an electromagnetic interference (i.e., EMI) filter, and a power factor correction circuit for converting AC power from the power source 102 to DC power rail V_rail. In another embodiment, the power source 102 is a DC power source, and the input stage 104 includes an EMI filter and an optional DC to DC converter, depending on the voltage of the power source 102 and the desired DC power rail voltage V_rail. The non-isolated driver circuit 100 with the full bridge rectifier further includes a controller 106, an inverter 108, and an output stage 110. The controller 106 receives an output current sensing signal I_sense from the output stage 110 and drives the inverter 108 as a function of the current sensing signal I_sense. The output current sensing signal I_sense is indicative of an output current provided to a load R_Load connected to the output stage 110. In one embodiment, the controller 106 alters a frequency of a drive signal provided to the inverter 108 as a function of the current sensing signal I_sense to maintain the output current at a target current.
The full bridge rectifier in the output stage 110 of the non-isolated DC power supply 100 of FIG. 1 electrically isolates the output (i.e., the output to the load R_Load of the output stage 110) from the input (i.e., circuit ground of the inverter 108) such that there is no return path from the output to the input. Thus, there is no return path for common mode noise to return to the circuit ground. If a frame attached to the load R_Load is grounded (i.e., connected to earth ground), then the common mode noise generated by the full bridge rectifier (i.e., D5, D6, D7, and D8) goes through earth ground to return to the circuit ground. This relatively large common mode noise could easily saturate a small common choke in the EMI filter of the input stage 104. As a result, the EMI filter of the input stage 104 needs to be relatively large and correspondingly expensive to stabilize operation of the non-isolated DC power supply 100.
Referring to FIG. 2, the output stage 210 of a non-isolated driver circuit 200 with a voltage doubler output includes a voltage doubler circuit having an input charge capacitor C_charge, an upper diode D3, a lower diode D4, and an output capacitor C1. The output stage 210 also includes a current sensing resistor R_I_sense in series with the load R_Load. The input charge capacitor C_charge is a charge capacitor that stores the energy from the resonant capacitor C_res of the inverter 108. The lower diode D4 is used to charge up the input charge capacitor C_charge. The upper diode D3 is used to pump the energy from the input charge capacitor C_charge to the output capacitor C1 and the load R_Load. The output capacitor C1 is used to filter out AC ripple at the output (i.e., across the load R_Load). Thus, the output is grounded (i.e., connected to ground via the current sensing resistor R_I_sense) and noise at the output has a direct path to return to the circuit ground. As a result, the non-isolated driver circuit 200 is more stable and quieter than the non-isolated driver circuit with full bridge rectifier 100 shown in FIG. 1.
The non-isolated driver circuit 210 with voltage doubler output has an inherent issue because only half of the resonant current from the resonant inductor Lres of the inverter 108 goes through the upper diode D3. Thus, to get the same amount of output current as the non-isolated driver circuit with full bridge rectifier 100, the non-isolated driver circuit with voltage doubler 200 has to put twice as much current through the resonant inductor Lres of the inverter 108. This relatively large current requires a larger and more expensive resonant inductor Lres as well as larger, more expensive inverter switches Q1 and Q2.
Referring to FIG. 3, a plot 300 of resonant inductor current 302 versus time is shown for the non-isolated driver circuit with voltage doubler 200 of FIG. 2 for a 700 mA output current (i.e., current delivered to the load R_Load). The resonant inductor current 302 in the inverter 108 peaks at 2.4 A.